INTRAMOLECULAR AROMATIC SUBSTITUTION
April 1970
139
Intramolecular Aromatic Substitution in Transition Metal Complexes GEORGEW. PARSHALL Central Research Department, Experimental Station, E. I . du Pont de Nemours and Company, Wilmington, Delaware 19898
Received November 24, 1969
A new aromatic substitution reaction, possibly related to classical electrophilic substitution, has recently emerged. In this reaction, an aryl C-H bond in a donor ligand of a transition metal complex reacts with the central metal atom to form a metal-carbon bond. The hydrogen originally attached to carbon adds to the metal to form a metal-hydrogen bond (eq 1)' or is eliminated as H+ (eq 2).2 This reaction is generally intramolecular and involves an ortho hydrogen bond of an aromatic N- or P-donor ligand.
monoalkylbenzylamines do not meta1at.e with Pd or P t salts, but instead form simple coordination complexes such as (CeH6CH2NH2)2PdClz.The orthosubstitution reaction involves proton elimination (eq 2), and it is promoted by proton acceptors such as tributylamine or excess benzyldialkylamine.2 However, the effect is probably kinetic rather than thermodynamic because l-phenylpyrazole undergoes metalation to give 2 even in dilute HC1 or HC104.3
2
CH2NR2 I
1
In the following discussion we shall first consider individual examples of this reaction with N-donor ligands and P-donor ligands and then attempt to relate these very diverse reactions. Finally, we shall consider extensions of this substitution reaction to intermolecular systems including metal-catalyzed substitution of benzene.
Nitrogen Donor Ligands The substitution of an aryl group which is part of an N-donor ligand molecule has been studied most extensively with palladium. The simplest example is that of eq 2, in which a benzyldialkylamine is treated with methanolic LizPdC14to give an o-palladiobenzene (1) complex.2 The reaction of a benzyldialkylamine with K2PtC14 proceeds similarly a1though more slowly to give the platinum analog of 1. Benzylamine and (1) G. Hata, H . Kondo, and A. Miyake, J . Amer. Chem. Soc., 90, 2278 (1968). (2) A. C. Cope and E. C. Friedrich, ibid., 90, 909 (1968).
3
The ortho-palladation reaction shows a strong tendency to form five-membered rings.2 In the series C6Hs(CH2)nN(CH3)2, in which n may be 0, 1, 2, or 3, only the benzylamine (n = 1) undergoes metalation. The other amines, which would form four-, six-, or seven-membered rings by ortho substitution, give only the complexes [C~H~(CHZ)~N(CH~)Z]ZP~C~Z. Similarly, l-naphthyldimethylamine, which could be attacked at either the 2 or 8 position, forms the five-membered structure 3 by reaction with LitPdC14. Azobenzene4reacts with divalent Pd and Pt salts to give dimeric ortho-metalation products (4) similar to those obtained from the benzylamines. These dimeric products (1-4) have been difficult to characterize structurally because they are insoluble in most common organic solvents. However, tractable monomeric derivatives are formed by treatment with amines4 and phosphine^.^ The phosphines cleave not only the chloride-bridge bonds as do amines, but also the N-Pd coordinate bond. A crystal structure determination5 on 5 demonstrated that a C-Pd bond had formed in the ortho position. This result is in accord with the earlier finding4 that LiAlD4 cleavage of the C-Pd bond in 4 gives o-deuteriohydrazobenzen e. The ortho-palladation reaction occurs with many symmetrically substituted a z ~ b e n z e n e s ,but ~ , ~some ~ ~ of the most interesting results are obtained from azo(3) 9. Trofimenko, private communication. (4) A. C. Cope and R. W . Siekman, J. Amer. Chem. Soc., 87,3272 (1965). (5) R. W. Siekman and D. L. Weaver, Chem. Commun., 1021 (1968).
GEORGE W. PARSHALL
140
5
4
benzenes bearing a para substituent on only one of the two aromatic rings.O In this situation, the choice of the ring to be substituted depends on electronic effects and provides some clue to the mechanism of the reaction. The results strongly suggest electrophilic substitution. The preference for attachment of the metal to the substituted ring in the product 6 falls in the order OCH3 > CH3 > H > C1, thus indicating metalation of the more electron-rich ring. With 4methoxyazobenzene only the substituted ring was attacked to give 6 (X = OCHs), but with 4-chloroazobenzene metalation of the unsubstituted ring predominated by a 3 : 1 margin.
LH$~
CGHj
Cl+ 6
7
The method of analysis of the isomers6 of 6 is interesting because it suggests synthetic applications for this reaction. The crude product, 6, obtained by reaction of the azobenzene with ethanolic NazPdCL, was treated with carbon monoxide (150 atm, 100') in ethanol to give the indazolone 7. This product, in turn, was degraded to give aniline and 5-X-2-aminobenzoic acid. The formation of 7 presumably occurs by insertion of CO into the C-Pd bond of 6 followed by displacement of Pd from N to give the indazolone ring. The selectivity of the palladation reaction for ortho substitution should make this chemistry useful for synthesis of compounds difficult to obtain by classical methods. It seems likely that an analogous ortho metalation of azobenzene by cobalt is a key step in the Co2(CO)e-catalyzed synthesis of indazolone from azobenzene and CO.* Analogous o-metaloazobenzene complexes have been prepared from nickelocene and azobenzene9 or substituted azobenzenes.'O Azobenzene reacts a t 135" (6) H. Takahashi and J. Tsuji, J. Orgunometal. Chem., 10, 511 (1967). (7) R. F. Heck, J . Amer. Chem. Soc., 90,313 (1968). (8) S. Horiie and S. Murahashi, Bull. Chem. SOC.Jup., 33, 247 (1960). (9) J. P. Kleiman and M. Dubeck, J . Amer. Chem. Soc., 8 5 , 1544 (1963). (10) Yu. A. Ustynyuk, I. V. Barinov, T . I. Voevodskaya, and N. 9.Rodionova, Abstracts, 4th International Conference on Organometallic Chemistry, Bristol, 1969, p J16.
Vol. 3
to form 8, presumably with elimination of cyclopentadiene. Compound 8 is soluble, apparently monomeric, and is surprisingly stable to air. This reaction, like that with palladium salts, is accelerated by electron donor substituents on the aromatic rings. The palladium analog of 8 has been prepared by treatment of the chloride-bridged dimer 4 with sodium cyclopentadienide.' Attempts to prepa.re an iron carbonyl derivative ad azobenzene by reaction with Fez(CO)gl* gave a rearranged product with no aryl-Fe bonds. l 2
8
In contrast to azobenzene, the isostructural Schiff bases such as X-benzylidenaniline react with Fe2(C0)9 to give ortho-metalated products. l 3 The crystal structure" of the p-toluidine derivative (9, Ar = p CH3CeHJ displays a nonplanar five-membered ring containing the ortho-bonded iron. Another i?e(C0)3 group bonds to one face of the ring; the CH2 of the ring is bent away from the second iron. This CH2 is evidently produced from the azomethine carbon of the Schiff base by transfer of an ortho hydrogen from the aromatic ring. When no such hydrogen is present in the starting material as in 2,6-dichlorobenzylideneaniline, complex formation does not occur. Schiff bases and other structural analogs of azobenzene react with PdCh complexes to give products strictly analogous to the o-palladioazobenzene complexes (4). A considerable series of N-benzylideneanilines have been treated with (CeH5CN)2PdCl2to give compounds such as 10, in which the palladium is bound to the benzylidene ring. l 4 Similarly, 2-phenylpyridine with NanPdCle gives 11 and 2-phenylquinolin e gives a benzo derivative of l l . 1 5
10
11
Phosphorus Donor Ligands Triarylphosphine and phosphite complexes of many group VI11 metals undergo intramolecular aromatic (11) M. M . Bagga, W. T . Flannigan, G. R. Knox, and P. L. Pauson,
J. Chem. Soc., C, 1534 (1969).
(12) P. E. Baikie and 0. S. Mills, Chem. Commun., 707 (1966).
(13) M. M. Bagga, W. T. Flannigan, G . R. Knox, P. L. Pauson, F. J. Preston, and R. I. Reed, J. Chem. SOC.,C, 36 (1968). (14) S. P. NIolnar and 31.Orchin, J . Orgunometal. Chem., 16, 196 (1969). (15) A. Kasahara, Bull. Chen. Soc. Jup., 41, 1272 (1968).
INTRAMOLECULAR AROMATIC SUBSTITUTION
April 1970
substitution. As with the N-donor ligands, ortho substitution occurs, but the role of the metal atom is more complex. For example, the substitution may involve a formal oxidation of the metal as in eq 3,1e More commonly the initial and final oxidation states of the metal are the same since, as in eq 4,”sls substitution is accompanied by elimination. However, in contrast to the N-donor ligand complexes studied to date, the elimination involves HZor alkane, rather than HC1.
12
0 13
14
(4)
15
141
intermediate may then undergo “reductive elimination”19 of CH4 to give the observed product, 15. Such an elimination of H and alkyl attached to the same metal atom is commonly accepted as a key step in the hydrogenation of olefins.20J1 Consistent with this postulated reaction sequence, pyrolysis of [(CBD5)3Pl3RhCH3 produces CHaD. The C-Rh bond in 15 undergoes many normal reactions of an organometallic compound,2o including cleavage and insertion. Carbon monoxide “inserts” into the C-Rh bond to give an acyl function, C-C(=O)Rh,’* as diagnosed by the appearance of infrared absorption a t 1620 cm-l. Phenol and HP cleave the bond to give CeHsORh[P(C8H5)3]3and HRh[P(C&)3]3, re~pective1y.l~The latter cleavage is apparently reversible, since use of Dz leads to extensive incorporation of deuterium into the triphenylphosphine ligand.18 Transition metal hydride complexes containing aromatic phosphorus ligands readily eliminate Hz on ortho substitution. This tendency is illustrated in the equilibrium between the triphenyl phosphite complexes 16 and 17. Although either HZ or HCl could be formed by ortho substitution of a phenoxy group, hydrogen is cleanly evolved when 16 is heated in an inert solvent.22 The ortho-bonded compound (17) is stable, but readily reverts to the hydride (16) on treatment with hydrogen. If 17 is treated with DZ rather than HP, the corresponding metal deuteride is formed and deuterium appears in the ortho positions of the phenoxy groups. The equilibrium between 16 and 17 is evidently facile because equilibrium is readily attained in the deuteration of the 24 ortho positions of 16 a t room temperature. Phenol added to solutions of 16 under Dz is also ortho deuterated, apparently by exchange with the phenoxy groups of triphenyl phosphite. In contrast to the rhodium complex 15, the ortho-bonded ruthenium species 17 does not readily undergo CO insertion.
The oxidative substitutions such as those of eq 3 are typical examples of the “oxidative addition” reactions recently recognized in transition metal chemistrylg since the metal undergoes an increase in both formal oxidation state and coordination number. For example, in eq 3, a four-coordinate, d8 iridium (1+) complex (12) is converted to a six-coordinate, de iridium (3+) compound (13) by addition of an ortho C-H bond to the metal.’ Although the gross reaction is simply an isomerization, the product is easily detected by the presence of characteristic Ir-H absorptions in the infrared and nmr spectra. Bennett and Milnerl6 have shown that the ortho substitution is promoted by electron-donating substituents on the aromatic ring. For the para-substituted phosphine complexes [(XC&t4)3P]31rC1, the rate of the reaction decreases in the series CHI > OCH3 > H >> F. A single ortho deuterium on each ring brings about a slight slowing of the reaction, corresponding to k ~ / of k ~about 1.4, but experimental problems prevent an accurate determination of this ratio. The acceleration of the reaction by electron-donor substituents could be due to either or both of two effects: (1) an enhancement of the electron density in the ring should promote electrophilic substitution as was noted for the N-donor complexes; or (2) enhanced electron density on the metal atom should promote “oxidative addition”l9 to the metal. The more common type of ortho substitution exemplified by reaction 4l7,l8may also involve oxidative addition. In the thermolysis of 14, which occurs even a t O”, the first step may be addition of an ortho C-H bond t o the metal atom to give a six-coordinate rhodium (3+) species very similar in structure to 13. This
The exchange of aromatic hydrogens with D2 is a useful tool for detecting a reversible reaction analogous to 16 $ 1 7 when the equilibrium concentration of the ortho-bonded species is very The appearance of hydrogen in the gas phase indicates the rate and extent of exchange. Nmr analysis of the aromatic ligand shows the position at which deuterium is introduced. The exchange of ortho hydrogens with Dz is rapid, even at room temperature with the hydrogenation catalyst [(CeH6)3P]3RuHCl. This compound cata-
(16) M. A. Bennett and D. L. Milner, J . Amer. Chem. Soc., 91, 6983 (1969). (17) W. Keim, J. Organometal. Chem., 14, 179 (1968). (18) W. Keim, ibid., 19, 161 (1969). (19) J. P. Collman, Accounts Chem. Res. 1, 136 (1968).
(20) G. W. Parshall and J. J. Mrowca, Advan. Orgummetal. Chem., 7, 157 (1968). (21) J. A. Osborn, F. H. Jardine, J. F. Young, and G. Wilkinson, J. Chem. SOC.,A , 1711 (1966). (22) G. W. Parshall, W. H. Knoth, and R. A. Schunn, J. Amer. Chem. Soc., 91,4990 (1969).
c1
GEORGE W. PARSHALL
142
VOl. 3
thesis of the complex from selectively deuterated liglyzes specific ortho deuteration of (C6Hj)~psince the ands. Metal-carbon bond formation has also been coordinated phosphines are in rapid equilibrium with postulated in platinum complexes of (C2H~)3P32 and free ligand in solution.22 The triphenyl phosphite (C~H~)ZPCH .~~ A ~ metal-aliphatic hydrogen intercomplexes [ (Cd%0)3P]&oH and [ (CsH50)3P]kRhH action is also implicated in the reaction of tri-o-tolylundergo statistical ortho exchange with Dz at lOO”, phosphine with RhCl3 to produce coupling of methyl but with the cobalt compound the exchange is inhibited groups to give the complex 20.34 by excess (C&0)3P. This inhibition suggests that dissociation of a ligand to give a “coordinatively unsaturated” complex is essential.22 The osmium complexes [ (CE,H~)~P]~(CO)OSHZ and [ (C6H5)3P]3(CO)OSHC1 exchange ortho hydrogen with D2 at 100-120°.23 (0-Tol)zP-RhP(o-Tol)2 The latter compound also exchanges aromatic hydroI gens with DzO at high temperature^.^^ The exchange c1 of Hz with a deuterated ligand as in [ ( C ~ D ~ ) ~ P ] ~ C O H ~ ~ ~ 20 is also a useful way to determine the extent and posiMechanism tion of exchange. The molecular nitrogen complex [(C&)SP]~RU(NZ)- The preceding examples of intramolecular subHz readily exchanges ortho hydrogens with Dz at 65’, stitution in aromatic N- and P-donor ligands illustrate but the ortho-substituted product is not isolable.22 both the broad scope of this reaction and the dearth The related iron compound, [ (CE,H~)~PCZH~]~F~(NZ)H~, of information about its mechanism. The observed evolves hydrogen in sunlight to give a stable orthosubstituent effects in the azobenzene complexes suggest bonded compound.26 It has been suggestedz7 that electrophilic attack of the aromatic ring by the metal earlier confusion as to whether the cobalt complex of nucleus. However, the corresponding information Nz is [ ( C G H ~ ) ~ P ] ~ C O (or NZ simply ) H [(CeHh)3P]aCo(Nz) is generally lacking for triaryl phosphite and phosphine is due to an equilibrium between the hydride and an complexes. Despite the many gaps in our knowledge, ortho-bonded complex enough is known to permit reasonable guesses about some possible intermediates in ortho substitution. [(C~H~M’I~CO(N~)H18 Hz Figure 1 shouis some plausible sequences for the ortho The hydride form of the compound has been conbonding of the ruthenium complex 16 and for the ortho clusively identified by crystallography2* and does insubstitution of azobenzene. The two sequences indideed exchange o-H with D2.22 The cobalt complex cate some of the similarities and differences expected. decomposes thermally with formation of benzenez9 In both sequences, the first significant interaction and, by inference and odor,23 diphenylphosphine. between the metal atom and the aromatic ring is in the This cleavage may occur by way of an ortho-substituted n-arene complex that results from step B. No direct triphenylphosphine complex (18) like that (19) reevidence for a n-arene complex has been found in the ported in the decomposition of [ ( C ~ H ~ ) ~ P ] Z P ~ ( C Z O ~ ) . ~ ~ systems described above, but many are known in transition metal chemistry. The closest parallel is the naphthalene complex 21 formed by reduction of the (CH3)2PCH2CH~P(CH3)2 ( D X P E ) complex of RuClz with sodium naphthalide. The reduction product displays chemical properties corresponding to the n-
+ +
18
19
I n a few instances, aliphatic hydrogens from phosphine ligands interact with coordinatively unsaturated metal atoms. The zerovalent complex [ (CH3)2PCH2CHzP(CH3)2]zRu is reported to exist in equilibrium with a Ru(I1) hydride which forms by transfer of a hydrogen from one of the methyl groups of the ligand.31 The source of the hydride H was determined by syn(23) G. W. Parshall, unpublished results. (24) L. Vaska, J . Amer. Chem. Soc., 86, 1943 (1964). (25) A. Sacco and M. Rossi, Inorg. Chim. Acta, 2, 127 (1968). (26) A. Sacco and M. Aresta, Chem. Commun., 1223 (1968). (27) A. D. Allen and F. Bottomley, Accounts Chem. Res., 1, 360 (1968). (28) B. R. Davis, N. C. Payne, and J. A. Ibers, J . Amer. Chem. Soc., 91, 1240 (1969). (29) A. Misono, Y .Uchida, and T . Saito, Bull. Chem. SOC. Jup., 40, 700 (1967). (30) D. M. Blake and C. J. Nyman, Chem. Commun.,483 (1969). (31) J . Chatt and J. M. Davidson, J . Chem. Soc., 843 (1965).
arene formulation 21, but its spectroscopic properties indicate hydride structure 22.31)35This system is particularly pertinent to the Ru complex of Figure 1 because the equilibrium 21 2 22 is exactly analogous (32) S. Bresadola, P. Rigo, and R. Turco, Chem. Commun., 1205 (1968). (33) M. Green, R. B. L. Osborn, and F. G. A. Stone, J . Chem. Soc., A , 3083 (1968). (34) M. A. Bennett and P. A. Longstaff, J . Amer. Chem. Soc., 91, 6266 (1969). (35) 5 . D. Ibekwe, B. T. Kilbourn, U. A . Raeburn, and D. R. Russell, Chem. Commun., 433 (1969). A crystal structure determination showed that, in the solid state, this compound exists as the 2-naphthylruthenium hydride (22).
April 1970
INTRAMOLECULAR AROMATICSUBSTITUTION PdC1,2-
+
Ar-N=N-CeHS
C1 Cl-bd-f
I
/
Ar-N=N
It
Jt I
(Cp,Hj0)2P-O
c1
/
ments in the Pd sequence may occur by either S N or ~ S N mechanisms. ~ A simple azobenzene complex of palladium analogous to that formed in step A has been isolated recently.38 It is readily converted to the ortho-bonded complex 4 by heating in solution. Steps C-E in the Pd complex more closely resemble those established in classical electrophilic aromatic substitutions than that proposed for the Ru complex. The reversible loss of HZ (step E) from the ruthenium compound is a common reaction of transition metal dihydride complexes.21 I n both sequences, step F restores the metal atom to its normal coordination number, six for divalent Ru, four for divalent Pd The palladation reaction of dialkylbenzylamines is formally analogous to a lithiation reaction discovered by H a u ~ e r . n-Butyllithium ~~ reacts with beneyldimethylamine to give the o-lithium compound 23.
Ar-N=N
It
-H+j/+H’
23
C1-Pd
1 9
Ar-N=N +ligand Jr-ligand
C1 dimer 4 (C6H50)tP-
143
6
17
Figure 1.
to the transformation effected by steps C and D. The mechanism of these steps is unknown, but the very small deuterium isotope effect noted in substitution of o-deuteriotriphenylphosphine suggests a three-center intermediate such as16
24
This derivative, in turn, has been used to prepare transition metal complexes such as 24.40 The formal analogy between palladation and lithiation, however, may not be valid mechanistically because there is evidence that contradicts an electrophilic substitution of aromatics by lithium.41 In addition, the analogy does not extend to the metalation of triphenylphosphine which, as we have seen, readily undergoes ortho substitution in several transition metal complexes. The reaction with n-butyllithium proceeds very slowly and gives mlithi~phenyldiphenylphosphine.~~ This failure to promote ortho lithiation could be ascribed to poor complexing of Li by the phosphine or to the unfavorable ring size involved in an intramolecular transition state. At any event, the lithiation is slow, comparable to that of benzene,41 and gives a different isomer from that predicted by our analogy. Intermolecular Substitution The still unresolved mechanistic question as to whether ortho metalation is promoted by a kinetic effect such as stabilization of a transition state or by a thermodynamic stabilization of the product has importance beyond the understanding of the mechanism. If
This type of carbon-to-metal hydrogen transfer should (37) The postulated dissociation of 16 is supported by the obbe facilitated by n-arene complex formation, but is not servation that excess triphenyl phosphite inhibits exchange with absolutely required. I n complexes such as [ (ceHs)aP]~- Dz. A similar inhibition was also noted in DZ exchange with [(CeHsO)aP]tCoH for which there is more direct evidence for ligand RuC12, one aryl hxdrogen approaches the metal atom dissociation. Dr. R. A. Schunn has found that the apparent molecvery closely (2.59 A) in the solid state.36 ular weight of the cobalt hydride in aromatic solvents falls from 1261 a t 37O to 1174 a t 80°, and to 1090 a t 110’. The postulated ruthenium and palladium reaction (38) A. L. Balch and D. Petridis, Inorg. Chem., 8 , 2247 (1969). sequences differ in many respects. Steps A and B in (39) R. L. Gay and C. R. Hauser, J . Amer. Chem. Soc., 89, 2297 (1967), and references cited therein. the Ru series correspond to an s N 1 replacement of (40) A. C. Cope and R. N. Gourley, J. Organometal. Chem., 8 , 527 phosphite by arene. 37 In contrast, ligand displace(1967). (36)
s. J. La Placa and J. A . Ibers, Inorg. Chent., 4,778 (1965).
(41) H. Gilman and J. W. Morton, Org. React., 8 , 258 (1954). (42) H. Gilman and G. E. Brown, J . Amer. Chem. Soc., 67, 824 (1945).
144
GEORGE W. PARSHALL
the effect of the donor substituent is simply to lower the activation energy for metalation, one might expect metalation of benzene itself to occur, although a t a much lower rate. If metalation of nonactivated aromatics does occur and can be promoted by catalysts, new patterns of substitution may become available. Indeed, some reactions of benzene do suggest the formation of metal-carbon bonds under mild conditions. When [ (C0)2RhC1]2 and tetraphenylporphine (tpp) are boiled in benzene, CsHsRhCl(tpp) is formed,43 apparently by substitution of rhodium on benzene. Exchange between aromatic C-H and DzO is catalyzed by PtC142- in CH3COOD solution, possibly by a mechanism44closely analogous to that proposed for the palladation of azobenzene in Figure 1. Similar exchanges with Au3+, Hg2+, T13+,Pd2+, and P t 2 + have been taken as evidence for the formation of arylmetal bonds with these ions.46 The expectation of new aromatic substitution chemistry has been realized in the reactions of palladium carboxylates with benzene derivatives. Palladium acetate and b e n ~ e n e , depending ~ ~ ' ~ ~ on reaction conditions, give either phenyl acetate or biphenyl. With toluene, the isomer distribution in the bitolyl that is formed suggests an electrophilic reaction to form a tolylpalladium complex (at 50°, ortho:meta: para is 8.7:26.3: 65.0) which decomposes t o give the observed products. In contrast to these reactions of the acetate, palladium propionate reacts with benzene to give cinnamic acid.47
L,RuHCl
VOl. 3
+
C2H,
27 L = (CeHj)3P
Figure 2.
may explain the rather puzzling hydrogenation of olefins in the absence of H2 by catalysts such as [ (CeH5)3P]~NiBrz.~~ A plausible mechanism that has been proposed48for this stoichiometric reduction process is illustrated in Figure 2. The illustration deals with [(C6H5)3P]3RuHC1(25) since the mechanisms of hydrogenation50 and of H-D exchangez2are better defined for this catalyst than for most others. The conventional hydrogenation path involves conversion Catalysis of the hydride to the alkylruthenium compound (26, Many of the triarylphosphine complexes that undergo x = 2 or 3) which then reacts with Hz to give alkane o-H exchange with Dz are active catalysts for the hyand regenerates a ruthenium hydride. However, drogenation of olefins. It now seems likely that the complex 26 may also undergo ortho substitution to form aryl hydrogens sometimes become involved in the hy27. This species may, in turn, evolve alkane and prodrogenation process. duce 28. Thus, hydrogenation of the olefin may occur In a study of catalytic reactions of olefins4*a series in absence of added hydrogen. If H2 is present in the of transition metal hydrides were allowed to react with system, 28 may be reconverted to the starting hydride D2C=CD2. Hydride complexes that showed activity (25) via 29. The involvement of ortho C-H bonds in hydrogenain the isomerization of olefins usually produced subtion is also apparent in the reaction of DZwith diphenylstantial amounts of CzD3H, C2D2Hz, and C2DH3. However, some hydrides such as [ (C6H5)3P]3RuHC1 acetylene reported recently.51 This reaction, which is catalyzed by a (P~)~R~C~~-(CH~)~NCHO-? and [(CGH50)3P]&oHgave partially deuterated ethanes in addition to the ethylenes. The total amount of combination, leads to the formation of o-deuteriohydrogen introduced into the gases exceeded that availtrans-stilbene (30). ortho substitution of the aromatic able from the M-H function. Since both complexes ring by rhodium provides a mechanism for hydrogen undergo exchange of their ortho hydrogens with D z , ~ ~transfer from the ring to the (Y carbon. it seems reasonable to propose that the "excess" hydrogen in the C2 products originated in the aryl groups. This donation of hydrogen from an aryl C-H bond (43) E.B. Fleischer and D. Lavallee, J. Amer. Chem. SOC.,89,7132 (1967). (44) R. J. Hodges and J. L. Garnett, J . Phus. Chem., 73, 1525 (1969), and references therein. (45) J. M. Davidson and C. Triggs, J . Chem. Soc., A , 1324 (1968). (46) J. M. Davidson and C. Triggs, ibid.,A , 1331 (1968). (47) S. Nishimura, T. Sakakibara, and Y. Odaira, Chem. Commun., 313 (1969). (48) R. A. Schunn, 155th National Meeting of the American Chemical Society, San Francisco, Calif ,, April 1968.
30
(49) H. Itatani and J. C. Bailar, J. Amer. Chem. Soc., 89, 1600 (1967). (50) P. S. Hallman, B. R. h,IcGarvey, and G. Wilkinson, J . Chem. SOC.,A , 3143 (1968). (51) P. Abley and F. J. McQuillin, Chem. Commun., 1503 (1969).
